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scache - Postfix shared connection cache server
scache [generic Postfix daemon options]
The scache(8) server maintains a shared multi-connection cache. This
information can be used by, for example, Postfix SMTP clients or other
Postfix delivery agents.
The connection cache is organized into logical destination names, phys-
ical endpoint names, and connections.
As a specific example, logical SMTP destinations specify (transport,
domain, port), and physical SMTP endpoints specify (transport, IP
address, port). An SMTP connection may be saved after a successful
In the general case, one logical destination may refer to zero or more
physical endpoints, one physical endpoint may be referenced by zero or
more logical destinations, and one endpoint may refer to zero or more
The exact syntax of a logical destination or endpoint name is applica-
tion dependent; the scache(8) server does not care. A connection is
stored as a file descriptor together with application-dependent infor-
mation that is needed to re-activate a connection object. Again, the
scache(8) server is completely unaware of the details of that informa-
All information is stored with a finite time to live (ttl). The con-
nection cache daemon terminates when no client is connected for
max_idle time units.
This server implements the following requests:
save_endp ttl endpoint endpoint_properties file_descriptor
Save the specified file descriptor and connection property data
under the specified endpoint name. The endpoint properties are
used by the client to re-activate a passivated connection
Look up cached properties and a cached file descriptor for the
save_dest ttl destination destination_properties endpoint
Save the binding between a logical destination and an endpoint
under the destination name, together with destination specific
connection properties. The destination properties are used by
the client to re-activate a passivated connection object.
Look up cached destination properties, cached endpoint proper-
ties, and a cached file descriptor for the specified logical
The scache(8) server is not security-sensitive. It does not talk to the
network, and it does not talk to local users. The scache(8) server can
run chrooted at fixed low privilege.
The scache(8) server is not a trusted process. It must not be used to
store information that is security sensitive.
Problems and transactions are logged to syslogd(8).
The session cache cannot be shared among multiple machines.
When a connection expires from the cache, it is closed without the
appropriate protocol specific handshake.
Changes to main.cf are picked up automatically as scache(8) processes
run for only a limited amount of time. Use the command "postfix reload"
to speed up a change.
The text below provides only a parameter summary. See postconf(5) for
more details including examples.
The maximal time-to-live value that the scache(8) connection
cache server allows.
How frequently the scache(8) server logs usage statistics with
connection cache hit and miss rates for logical destinations and
for physical endpoints.
config_directory (see 'postconf -d' output)
The default location of the Postfix main.cf and master.cf con-
How much time a Postfix daemon process may take to handle a
request before it is terminated by a built-in watchdog timer.
The time limit for sending or receiving information over an
internal communication channel.
The maximum amount of time that an idle Postfix daemon process
waits for an incoming connection before terminating voluntarily.
The process ID of a Postfix command or daemon process.
The process name of a Postfix command or daemon process.
The syslog facility of Postfix logging.
syslog_name (see 'postconf -d' output)
The mail system name that is prepended to the process name in
syslog records, so that "smtpd" becomes, for example, "post-
smtp(8), SMTP client
postconf(5), configuration parameters
master(8), process manager
syslogd(8), system logging
Use "postconf readme_directory" or "postconf html_directory" to locate
CONNECTION_CACHE_README, Postfix connection cache
The Secure Mailer license must be distributed with this software.
This service was introduced with Postfix version 2.2.
IBM T.J. Watson Research
P.O. Box 704
Yorktown Heights, NY 10598, USA
111 8th Avenue
New York, NY 10011, USA